Seismic behaviour of stiffness irregular steel frames under mainshock–aftershock

Abstract

Generally, the buildings located in earthquake-prone regions are subjected to a sequence of earthquakes. The sequence of earthquakes is termed as mainshock–aftershock (MS–AS). Most of the time the rehabilitation of structure before the occurrence of aftershock cannot be carried out due to a short interval of time between mainshock and aftershock results in additional damage to the structure. The irregular distribution of mass, stiffness and strength along the height of the building may exhibit the poor seismic performance of the structures. The present study examined the influence of stiffness irregularity on seismic demands of 3-, 6- and 9-storey steel moment-resisting frames by comparing the mean seismic demands on reference regular frames under mainshocks and MS–AS seismic sequences. Stiffness irregular frames are obtained by modifying the stiffness at three different locations (bottom, middle and top storey) along with the height of reference regular frame. MS–AS seismic sequences are generated by using a randomized approach. The comparison between reference and stiffness irregular frames shows that the effect of stiffness irregularity on the height-wise variation of interstorey drift ratio is significant. Results of the effect of MS–AS seismic sequence show that aftershock increases the structural responses of both reference regular and stiffness irregular frames.